Pad or transfer-pad printing is a well establish commercial method for transferring a wet coating in the form of a thin design to another surface, typically a dimensionally altered shaped surface such as one having a rounded or irregularly shape. It differs from printing in that it does not rely on a typeface in transferring the design. Pad printing has been used commercially for many applications. It has been used to print patterns on light bulbs, watch faces, golf balls; form printed circuits on dielectric substrates; imprint bar codes on a variety of surfaces; and add patterns to a contact lens mold (see copending U.S. applications Ser. Nos. 222,709, filed Jul. 21, 1988, and 329,431, filed Mar. 28, 1989) or on a contact lens (see U.S. Pat. No. 4,582,402, patented April 15, 1986, U.S. Pat. No. 4,704,017, patented Nov. 3, 1987, U.S. Pat. No. 4,946,269, and U.S. Pat. No. 4,946,269).
There are a number of transfer-pad printing systems being sold for such commercial applications. .sup.1 Though these system are automated and geared to accurately transfer the pattern from a printing block (cliche) that contain the wet coating design to be transferred, a critical step in the process involves accurately picking up the coating design from the cliche, and not altering the design while it lays on the pad prior to transfer coating to a receiving printing surface onto which the design is to be imprinted. FNT 1 For example, such systems are obtainable from Trans Tech America, Inc., Schaumburg, Ill. 60193; Markem Corporation, Keene, N.H. 03431; TampoPrint GmbH, Munich and Stuttgart, Germany.
There are a number of patents that describe apparatus suitable for transfer-pad printing. They include the following U.S. Pat. Nos.: 4,060,031; 4,282,807; 4,615,266; 4,779,531; 4,803,922 and 4,856,670. These apparatus are generally applicable to this invention.
A typical transfer-pad (also called a "tampon.") is made of low durometer rubber. The softness and compressibility of such rubber pads allows them to deform under mild pressure to be gently superimposed onto the coating design on the cliche. As a result, the pad compresses as pressure is applied to the cliche and this allows the pad to deform, to present a larger surface at the cliche-pad interface, thereby enshroud the coating design and pick up the design. For example, a design is engraved by etching into the cliche. A flowable coating composition is knife (doctor blade) coated onto the surface of the cliche, to fill up the engraved design with it. Then the tampon is caused to come into contact with the cliche's design, and as the pad is deformed and compresses over the coating composition on the engraved surface of the cliche, the surface of the pad contacting the surface of the cliche temporarily adopts the shape of the cliche's surface. If the pad is sufficiently soft and compressible, a portion of it that is contiguous to the coating design, will dwell within the engraved surface forming the design coating pattern on the cliche. This causes the flowable coating composition in the etched pattern on the cliche to make contact with the pad surface and if it sufficiently wets it, the pad will pick up coating replicating the design quite accurately. Before the coating dries on the pad surface, it is transferred to the printing surface.
Reproducing the design on a cliche on another substrate is dependent upon the ability of the coating composition deposited in the cliche to effectively wet the surface of the transfer-pad. If the coating composition, be it ink or paint, does not properly wet the pad's surface, then two things can happen. One, the pad will not completely pick up the coating in the design provided in the cliche. Second, the pad may pick up the coating design, by suction of a sufficient mass of the coating, but prior to transferring the design to the desired surface, the design breaks up (or beads up) on the pad because of an incompatibility between the coating and the pad's surface.
Another factor in transfer-pad printing that cab adversely affect the ability to properly transfer a flowable coating design to a printing surface, involves aging of the transfer pad. Over a period of time, the surface of the transfer-pad should pick up in its pores small amounts of the coating and/or its solvent components. With time, enough of a concentration of the coating should become imbedded in the surface of the pad so that the surface area of the pad is increased, or solvent attacks of the pad and permanently increases the pad's surface area, or a combination of these actions occurs. This added surface can create sites amenable to oxygen absorption leading to chemisorption of oxygen over the pad's surface. The result could change the surface characteristics of the pad sufficiently to adversely affect the wettability of the pad by the coating composition, even though the coating composition properly wets a new pad surface.
A popular transfer-pad rubber material is silicone rubber. Such rubbers are low durometer poly(dimethylsiloxane), generally of the RTV (room temperature vulcanization) type. RTV silicone rubbers are frequently made by the reaction of a difunctional dimethylsiloxane oil of the formula: ##STR1## wherein X is a hydrolyzable or condensable group such as halogen, alkoxy, aroxy, amino and hydroxyl; with a tri- or tetrafunctional silane of the formula: EQU RSi X.sub.3
wherein R may be X or organic bonded to the silicon by a carbon to silicon bond.
Silicone rubbers are notoriously well known to resist wetting by water. It should be noted that new silicone tampons typically contain silicone oil that exudes from the pad. That oil is recommended to be removed before the pad is used. This can involve wiping the pad, or washing it with a degreaser. Over time and use, the tampon becomes dirty and requires pad cleaner or plastic tape. Pad cleaners are stated to be only mildly aggressive and will not excessively dry out the silicone pad. Storage of the pad is proper when the pad is cleaned with a pad cleaner and then coated with a oil that penetrates into the pores of the pad.
It has been determined that latex coating compositions are not effectively transfer-pad printed because the desired design is not efficiently or accurately transferred to the printing surface. Latex coating tend to bead up on the tampon's surface, breaking up the design sufficiently that the resultant imprinted product is a poor replication of the original design.
It would be desirable to use latex coatings in pad transfer printing since they contain less volatile organic compounds (VOCs), and therefore, they are more environmentally desirable. Much of a latex coating composition is water. As compared to conventional solution coatings and inks, they contain much less VOCs and their use generates less VOC pollutants.
In the case of solution coatings or inks, they appear to be affected by aging of the silicone rubber transfer-pad. Over time and use, silicone transfer-pads become worn and present a more oxygenated surface that alters the pad's surface wetting characteristics. Such a pad will not efficiently or accurately transfer a replicated pattern of a solution based coating or ink composition to the desired surface.
Typical solution inks and coatings comprise an oleoresinous or oleophilic coating vehicle dissolved in solvents and in which dyes and/or pigments are dispersed. To the extent the surface of the tampon is oleophilic, such inks and coating are sufficiently compatible that they readily and effectively transfer from the cliche to the pad's surface and are retained thereon without loss of print definition, i.e., replication of the cliche's design. However, when the pad's surface ages, and becomes less oleophilic, the ink's or coating's compatibility with the surface is measurably reduced. This does not mean the pad's surface is wholly oleophilic. It merely means that the level of oleophilicity has been reduced to the extent that the ink or coating is not as wholly compatible as it was prior to the aging phenomena.
This problem is especially acute when the design being printed requires accurate and essentially complete replication on the printing surface. For example, a printed circuit requires that the pattern of the design be uniform and non-interrupted. A cosmetic contact lens requires that the design from lens to lens be uniform so that the right and left eyes appear the shine, even if the wearer has to replace one of a set of lenses. In addition, the cosmetic appearance of a contact lens will be dependent upon an accurate reproduction on the lens or in the lens, of the original design.
These problems are magnified in the case of automated pad printing system since the value of the system is generally dependent upon the extent of its on-line operation. Down time in the operation of the system has a material affect on the economics of the printing operation. Stopping of printing to correct inaccurate printing or replace tampons can be a serious economic penalty in such operations.